I thought I understood EM pretty well, but I've been thinking about it a bit lately and realised that I don't know as much as I thought I did. Here are a couple of questions I'd like answered, if anyone would oblige:

First, interaction (or lack thereof) between light and electric/magnetic fields. If light is a traveling disturbance in EM fields, then shouldn't it be deflected by such fields? I don't recall ever witnessing or hearing about such a phenomenon, so I wonder why it doesn't occur. What about static vs. dynamic (changing) electric or magnetic fields? (In the latter case, we're talking photons interacting with photons, so I can at least picture that as interference maybe -- but what about deflection?)

Second, potential energy. Imagine a permanent magnet surrounded by some iron filings at some distance from it. If you let the filings go, they will accelerate toward the magnet and slam into it, releasing energy as heat. This indicates the initial arrangement has potential energy in addition to energy inherently present in matter (as e=mc^2.) However, what if before I let go of the filings I oxidize them? Now they are no longer magnetic, so the potential energy disappears. Where did it go?? Isn't energy supposed to be conserved? In case I oxidized the filings after they've slammed into the magnet, I imagine I'd get the same energy out of the exothermic reaction -- or will I?

Joeman,

Apparently you know even less than I do.

The sort of polarity you're talking about is applicable to electric fields, not to magnetic fields. Magnetic fields interact with magnetic fields (e.g. magnets attracting each other.) Magnetic interactions are not the same thing as electrostatic interactions. Photons are electric and magnetic fields oscillating orthogonally, so at certain points in their phase they have a negative electrical charge while at other points they are positive. Similar for their magnetic component; it flips direction and changes magnitude.

With respect to iron filings, they are attracted to a magnet because they are magnetizable (they've got free electrons zipping between adjacent atoms, creating internal currents that in turn evoke magnetic fields; external magnetism restructures these internal currents so as to align their magnetic fields with the external field but opposite to it in orientation; this causes the two magnetic fields to exert an attractive force.) This is not the same thing as electrostatic attraction you seemed to imply. Iron oxide (rust), on the other hand, is not magnetizable (at least not your typical rust; magnetite is the exception.)

Originally posted by overdoze
Joeman,
Apparently you know even less than I do.

That is probably true. I have been out of school too long. Although I am a RF engineer and deal with EM daily.


The sort of polarity you're talking about is applicable to electric fields, not to magnetic fields. Magnetic fields interact with magnetic fields (e.g. magnets attracting each other.) Magnetic interactions are not the same thing as electrostatic interactions. Photons are electric and magnetic fields oscillating orthogonally, so at certain points in their phase they have a negative electrical charge while at other points they are positive. Similar for their magnetic component; it flips direction and changes magnitude.

EM waves are not always TEM waves. There are quasi-TEM waves and sometimes EM waves are not orthogonal at all. At near field, I don't believe EM waves are TEM waves. My last post is perhaps a little retarded, but if you send two EM waves of different frequencies out there in space, they superimpose but one don't deflect from another. When I think of EM waves, I think of E first because that is why I mentioned polarity. All I can say is, perhaps you think too much? :D


With respect to iron filings, they are attracted to a magnet because they are magnetizable (they've got free electrons zipping between adjacent atoms, creating internal currents that in turn evoke magnetic fields; external magnetism restructures these internal currents so as to align their magnetic fields with the external field but opposite to it in orientation; this causes the two magnetic fields to exert an attractive force.) This is not the same thing as electrostatic attraction you seemed to imply. Iron oxide (rust), on the other hand, is not magnetizable (at least not your typical rust; magnetite is the exception.)

Ya ya ya. I already know. I am rusty. Maybe I am just stupid. I had to delete my last post because it was embarrasing :rolleyes:

I thought Ironoxide (FeO) was usually caused in condunction with water. Which as you know is H<SUB>2</SUB>O.

I'm not sure if I'm correct, but I speculate that because Hydrogen is a single weak bond to a molecule, it would eventually break because of a particular polarity, cause the "Rust" to occur.

It's possible that the breaking of these bonds causes a De-entanglement of Fe aligning, couple with Oxygen it would produce inertia.

From what I remember on the subject and from a dictionary, you might want to check Magnetite. (Noted as Magnetic Iron Oxide)

Magnetite was kind of found by the Egyptians, and the Greeks.
It was said a Greek found it one day when he was walking across some rock, and his sandles became stuck to it, because the nails were attracted.

In Egypt they use to create offerings, of floating boxes, where they used the magnetite for floating things.

I'm not sure if it was a Persian city, but one city had it's vast gates made from Magnetite. It was said that no army could pass through the city's gate, Purely because any metalic armour or weaponry would stick to the gates.

Also to make a permanent magnetic there are two ways:

1: You can take a permanent magnetic and stroke a piece of iron with either pole.
2: When smithing metal, you could face the object north and if correctly tempered the objects magnetic structure aligns, to create a permanent magnet.
[I had a theory that a mythical sword of King Arthur fame, might have just been tempered this way. As it would have stuck solidly in a rock that was filled with Iron ore, or would have increased a stricking blow against armour. Although he would have to use his foot to pry it off afterwards.]

(Magnetite stuff from memory of a BBC documentary)

As for electromagnetics displacing photons, it's actually called "Electromagnetic stiffening", the method pretty much relies on Thermodynamics (Okay, Thermodynamics means heat, but the understanding of energy pretty much relates to the fact that heat exists at somepoint)

In fact MIT did do an experiment where they slowed a photon down by stiffening spacetime (namely the used alot of EM fields.)

Joeman,

No biggie. I don't think any less of you. :)

Just being rusty doesn't make you a retard. I might get rusty one day too. But not before I figure this shit out; it's really beginning to bug me now.

You're right, I do think too much. :p

Stryder,

Wow, didn't know about this "stiffening". I'll have to read up on it; thanks!

But with respect to iron and magnets, I just have to say: huh? One way to easily oxidize iron is to just set it on fire in a pure-oxygen atmosphere (yeah, in pure oxygen iron actually combusts.) And my real question wasn't about the nature of ferromagnetism, but about conservation of energy. So, any thoughts on that?

Hi Overdoze,

"First, interaction (or lack thereof) between light and electric/magnetic fields. If light is a traveling disturbance in EM fields, then shouldn't it be deflected by such fields? I don't recall ever witnessing or hearing about such a phenomenon, so I wonder why it doesn't occur. What about static vs. dynamic (changing) electric or magnetic fields? (In the latter case, we're talking photons interacting with photons, so I can at least picture that as interference maybe -- but what about deflection?)"

You say that light is a disturbance of electric and magnetic fields, of what fields ? I'd rather say that in the wave-description of light, light is in fact an electric and magnetic field propagating through space. You then describe them using Maxwell's equations and you get a complete uniformity between light and electromagnetical fields.

Using Maxwell's equations, an electric field (let's take it to be dynamic = radiowaves for example) is not different from light: they are both electromagnetic waves, described by the same equations of motion. I have no clue on how light would relate to the various multipoles of ELM radiation, perhaps someone would like to share his/her thoughts on this ?

Interference between waves can only occur if both sources are coherent. I don't think this will be the case for a lightsource and a source of an electric field. Perhaps this is why no interference or interaction occurs between a lightwave passing in an electric field. (Just guessing here, electrodynamics was not one of the things I was good at ;)).

"Second, potential energy. Imagine a permanent magnet surrounded by some iron filings at some distance from it. If you let the filings go, they will accelerate toward the magnet and slam into it, releasing energy as heat. This indicates the initial arrangement has potential energy in addition to energy inherently present in matter (as e=mc^2.) However, what if before I let go of the filings I oxidize them? Now they are no longer magnetic, so the potential energy disappears. Where did it go?? Isn't energy supposed to be conserved? In case I oxidized the filings after they've slammed into the magnet, I imagine I'd get the same energy out of the exothermic reaction -- or will I?"

Interesting question. From a classical point of view I think you can only argue that this energy was lost in the oxidation proces. If you consider the two distinct situations (imagine you have two seperate experiments) where in one experiment you have the filings unoxidized, and in the other they are oxidized, then there indeed is a difference in total energy. The only difference between the two experiments would be the oxidation, hence that must be the source of the difference in energy.

On the other hand... The above explanation was a mechanical explanation where the potential could have been anything. I am not sure if you can associate a potential with a magnetic field, but we will assume for a second that you can.
If you take the potential energy to be from a magnetic field, then there is no difference in energy between the two systems: if I am not mistaken, then for magnetical and electrical systems the potential energy is in fact the energy contained in the field (I don't have the expressions for the field energy of a magnetic field here, but I'd bet it would work out to be the potential energy of the problem). I suspect that we would have to move into the area of solid state physics (the interaction between waves and matter) to account for the difference in energy in this situation, but unfortunately, I never got into that.

Anyway, just some thoughts ;)

Bye!

Crisp

overdoze,

I spent a few years studying electric and magnetic fields so I'll give it a shot......

First, interaction (or lack thereof) between light and electric/magnetic fields. If light is a traveling disturbance in EM fields, then shouldn't it be deflected by such fields? I don't recall ever witnessing or hearing about such a phenomenon, so I wonder why it doesn't occur. What about static vs. dynamic (changing) electric or magnetic fields? (In the latter case, we're talking photons interacting with photons, so I can at least picture that as interference maybe -- but what about deflection?)

The reason that light is not influenced by magnetic fields is because the light's frequency is too high. If you were to look at a light photon microscopically, you would see the photon move towards the magnetic field of a magnet as the photon's magnetic field is pointing towards the magnet. However, the magnetic field of a photon oscillates, so when it's magnetic field is pointing away from the magnet, it will move away from the magnet. In other words, since the oscillating magnetic field of the light photons average out to zero in a very short period of time, magnetic fields will have no effect on them.

Second, potential energy. Imagine a permanent magnet surrounded by some iron filings at some distance from it. If you let the filings go, they will accelerate toward the magnet and slam into it, releasing energy as heat. This indicates the initial arrangement has potential energy in addition to energy inherently present in matter (as e=mc^2.) However, what if before I let go of the filings I oxidize them? Now they are no longer magnetic, so the potential energy disappears. Where did it go?? Isn't energy supposed to be conserved? In case I oxidized the filings after they've slammed into the magnet, I imagine I'd get the same energy out of the exothermic reaction -- or will I?

The magnetic field of a permanent magnet has no potential energy. Only when a iron filings come close to the magnet, does potential energy begin to exist. In other words, there has to be at least two magnetic objects for potential energy to exist in a magnetic field.

Basically, what happens is that the iron fillings become tiny magnets due to magnetic induction caused by the permanent magnet. This is caused by the electrons in the atoms of iron aligning to the magnetic field of the magnet. When this happens, potential energy is created, and an attractive force is created.

When you heat up iron filings they become iron oxide. Since the electrons that used to align with the magnetic field are now in different orbits(as a result of the chemical bond between oxygen and iron), their alignments are no longer influenced by the magnetic field of the permenant magnet. As a result, they don't align to the permanent magnet's magnetic field, and don't become magnetized. Therefore, no potential energy exists in this situation because there is only one magnetic object present.

Tom

Originally posted by overdoze
First, interaction (or lack thereof) between light and electric/magnetic fields. If light is a traveling disturbance in EM fields, then shouldn't it be deflected by such fields?


No, there is no such interaction in either QED or semiclassical EM field theory. EM fields couple to charges and currents, not to photons.

However, there is an interaction predicted by General Relativity, since both matter and energy densities go into the stress tensor, both are expected to cause spacetime to curve. The nonzero energy density in your scenario would result in such a curvature, however slight.


What about static vs. dynamic (changing) electric or magnetic fields? (In the latter case, we're talking photons interacting with photons, so I can at least picture that as interference maybe -- but what about deflection?)


Photons do not interact with photons directly. However, two photons can both produce a pair of virtual particles which in turn interact, giving the illusion that the photons themselves are interacting. Jackson's book Classical Electrodynamics gives a nice illustration in the Introduction.


Second, potential energy. Imagine a permanent magnet surrounded by some iron filings at some distance from it. If you let the filings go, they will accelerate toward the magnet and slam into it, releasing energy as heat. This indicates the initial arrangement has potential energy in addition to energy inherently present in matter (as e=mc^2.) However, what if before I let go of the filings I oxidize them? Now they are no longer magnetic, so the potential energy disappears. Where did it go??


You would have to do work to oxidize the filings, and the energy transfers involved in that process would account for the "missing energy".

Tom2,

I think I believe you about the filings; it makes a lot of sense. Thanks for solving that one for me!

About photons, I find Tom's (Prosoothus) explanation a little better; though I may be missing something major. Reason is, of course there is nothing for EM fields to couple to with respect to a photon; the photon itself is merely a phenomenon of the fields themselves. On the other hand, if the photon consists of oscillating EM fields, then as it moves past a point EM anisotropy it should be affected assymmetrically. Which leads me to conclude that long-wavelength radiation (e.g. microwaves) ought to be measurablly deflected by strong EM fields (and the direction of deflection would depend on the phase.) Anyone out there with a maser and a Van de Graaf generator? :)

Stryder,

I've tried to find some info on "electromagnetic stiffening" but have been unsuccessful so far. If you would be so kind, could you perhaps point out a couple of resources? Thanks much!

Originally posted by overdoze
Tom2,

I think I believe you about the filings; it makes a lot of sense. Thanks for solving that one for me!

About photons, I find Tom's (Prosoothus) explanation a little better; though I may be missing something major


His explanation is wrong.

Originally posted by Prosoothus:
The reason that light is not influenced by magnetic fields is because the light's frequency is too high.

The non-interaction of a photon with an EM field has nothing to do with the frequency of the photon. If the photon frequency were made arbitrarily small, there would still be no interaction, apart from the general relativistic one I mentioned earlier.


Originally posted by Prosoothus:
If you were to look at a light photon microscopically, you would see the photon move towards the magnetic field of a magnet as the photon's magnetic field is pointing towards the magnet. However, the magnetic field of a photon oscillates, so when it's magnetic field is pointing away from the magnet, it will move away from the magnet. In other words, since the oscillating magnetic field of the light photons average out to zero in a very short period of time, magnetic fields will have no effect on them.


1. Photons do not have magnetic fields.
2. Magnetic fields do not exert forces on other magnetic fields.

Originally posted by Tom2
The non-interaction of a photon with an EM field has nothing to do with the frequency of the photon. If the photon frequency were made arbitrarily small, there would still be no interaction, apart from the general relativistic one I mentioned earlier.


I'll grant you the GR interaction, for now (of course, I don't know if this has ever been experimentally verified; sounds like the effect would be too difficult to detect.)


1. Photons do not have magnetic fields.
2. Magnetic fields do not exert forces on other magnetic fields.

But photons are an oscillating EM entity. Which would imply that if there is a stationary, massive source of an EM anisotropy a passing photon would exert force on that source by interacting with its EM field. According to conservation of momentum, an opposite deflection would have to apply to the photon.

Originally posted by overdoze
I'll grant you the GR interaction, for now (of course, I don't know if this has ever been experimentally verified; sounds like the effect would be too difficult to detect.)


As you suspect, the GR interaction is currently an unverified prediction.


But photons are an oscillating EM entity. Which would imply that if there is a stationary, massive source of an EM anisotropy a passing photon would exert force on that source by interacting with its EM field. According to conservation of momentum, an opposite deflection would have to apply to the photon. [/B]

Photons are not miniature versions of electromagnetic waves. They have no EM fields of their own, and even if they did, they would not be attracted to another EM field. Now, photons do couple to charges, so you are right in saying that a photon would interact with a source--but only if the photon struck the source. You asked if the photon would be deflected by a field, and the answer is "no, at least not electromagnetically". Now you're addressing the question of whether or not a photon can interact with a charge, and the answer is "yes", but it is a different question.

Originally posted by Tom2
Now you're addressing the question of whether or not a photon can interact with a charge, and the answer is "yes", but it is a different question.

Would this mean it might be possible to deflect photons with static electricity, or are you still lacking the photons striking into anything?

Overdoze and Tom2,

If I wanted to test if photons interact with magnetic fields, I would use a device that is similar to overdoze's maser and Van De Graaf generator. Although, I would use, instead of a Van De Graaf generator, a high frequency electromagnet.

The closer the frequency of the electromagnet to the frequency of the microwaves from the maser, the greater the effect on the microwaves (assuming the synch is adjusted).

Tom

Tom2,

Photons are not miniature versions of electromagnetic waves. They have no EM fields of their own, and even if they did, they would not be attracted to another EM field.

If that's the case, what's the difference between coherent and incoherent light?

Tom

Re The iron filings question.

Ignoring the potential energy, Iron filings moving to the magnet part and leaving the loss of a magnetic field part.

The answer is that when you oxidise the iron. The iron will cool. Well actually get less hot. See magnetic cooling - used for producing close to absolute zero.

The reason is that the magnetism creates a degree of order in the iron. Question when a magnetism disapears like this the entropy increases and comes from ? Answer - by cooling.

This is your "missing" energy.